Grain-oriented electrical steel sheets are material mainly used as the iron core of a transformer, and from the perspective of achieving high efficiency of a transformer, grain-oriented electrical steel sheets are required to have, among other material properties, low iron loss properties.
Therefore, normally, a base film mainly composed of forsterite is formed on the surface of the steel substrate of the steel sheet during final annealing, and during or after flattening annealing, coating (insulating tension coating) mainly composed of phosphate and colloidal silica is applied and baked thereon for the purpose of achieving insulation and applying tension to the steel sheet as a product. The tension applied to the steel sheet by such base film and insulating tension coating improves iron loss properties.
Further, in order to reduce iron loss, it is important to highly accord secondary recrystallized grains of the steel sheet with the (110)[001] orientation, i.e. the so called “Goss orientation”. However, it is known that if the secondary recrystallized grains are caused to accord with this orientation too much, the iron loss ends up increasing.
Therefore, to address this issue, a technique has been developed to apply strains and grooves to the surface of a steel sheet to subdivide the width of a magnetic domain to thereby reduce iron loss, which is a magnetic domain refining technique. Among other magnetic domain refining techniques, non-heat resistant magnetic domain refining treatment is known to produce linear strain regions in a steel sheet to narrow magnetic domain widths and, although the effect is canceled by strain relief annealing, this treatment tends to have a more significant iron loss reducing effect compared to heat resistant magnetic domain refining treatment. Therefore this treatment is suitable for manufacturing low iron loss grain-oriented electrical steel sheets.
As methods for performing non-heat resistant magnetic domain refining treatment, methods using a laser beam, plasma flame, electron beam or the like are industrially used because of their high productivity.
As a method of such non-heat resistant magnetic domain refining treatment, for example, PTL1 (JPS57-2252B) proposes a technique of irradiating a steel sheet with a laser beam after final annealing to apply high-dislocation density regions to a surface layer of the steel sheet, to thereby narrow magnetic domain widths and reduce iron loss of the steel sheet. Further, magnetic domain refinement techniques using laser irradiation have been improved since PTL1, and grain oriented electrical steel sheets having better iron loss properties are being produced (see for example, PTL2 (JP2006-117964A), PTL3 (JPH10-204533A), and PTL4 (JPH11-279645A)).
As a technique of reducing iron loss by improving forsterite films, a technique of fixing Ti as TiN in the forsterite film is disclosed in PTL5 (JP2984195B).
Similarly, as a technique of reducing iron loss, a technique of specifying the contents of Ti, B, and Al in the forsterite film is disclosed in PTL6 (JP3456352B).
Further, PTL7 (JP2012-31512A) discloses a technique of controlling the N content in the base film to 3% or less and appropriately controlling the Al content and Ti content in the base film so that the iron loss after laser irradiation can be effectively reduced.
Further, PTL8 (JP2012-31518A) discloses a technique of preventing detachment of the forsterite film which tends to occur when performing non-heat resistant magnetic domain refining treatment.